Polyurethanes adhesives are a unique urethanes product group that vary widely in composition and are used in many different applications and market segments. Typical product forms include reactive types such as 1-component, 2-component and hot-melt compositions, as well as non-reactive types such as solvent-borne, water-borne and hot-melt compositions, among others.
Polyurethane adhesives are normally defined as those adhesives that contain a number of urethane groups in the molecular backbone of a polymer comprising the adhesive or which are formed during use, regardless of the chemical composition of the rest of the chain. Thus a typical urethane adhesive may contain, in addition to urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea and allophonate groups. An isocyanate group reacts with the hydroxyl groups of a polyol to form the repeating urethane linkage. Isocyanates will react with water to form a urea linkage and carbon dioxide as a by-product. Linear polyurethane adhesives may be obtained by using compounds with two reactive groups such as diisocyanates and diols. When polyols with three or more hydroxyl groups (i.e. a functionality of 3 or more) are reacted with an polyisocyanate, or when isocyanates with three or more isocyanate groups are reacted with a polyol the resulting polymer is crosslinked. In reaction systems where there is an excess of isocyanate, crosslinking reactions may occur. Often, excess isocyante in the composition reacts with atmospheric water or moisture contained in the substrate.
One component adhesives are usually viscous liquid isocyanate-terminated pre-polymers at room temperature. They set by reaction of the free isocyantes groups with atmospheric moisture or with moisture contained in the substrate to form polyurea groups. They typically do not require mixing with other components before curing. The prepolymers are prepared by reacting an excess of isocyanate with polyols. If the functionality of the prepolymer is greater than two the cured film will be chemically crosslinked.
Two component polyurethane adhesive compositions generally comprise components that are liquids or pastes at room temperature before they are mixed together. The first component of the composition comprises a polyol and other ingredients, such as chain extenders, catalysts, blocking agents and other additives as desired. The second component comprises monomeric, polymeric or prepolymeric polyisocyanate. In order to make a bond, the two components of the adhesive are fully mixed together and the composition is then applied to a substrate. The mixed composition then initiates cure and develops bonding strength while transforming into a solid form. The curing reaction takes place between the free isocyanate groups and the active hydrogens from the polyol. If there are excess free isocyanate groups after the main curing reaction, the excess free isocyanate groups are cured by ambient or surface moisture from the substrates. The isocyanates and polyols employed may have a functionality of two or higher to provide crosslinking in the adhesive.
Reactive hot melt adhesives are characterized as a readily meltable polyisocyanate polyurethane (NCO prepolymer) which is usually solid or highly viscous at room temperature. They set both physically by cooling and chemically by reaction with atmospheric moisture. Depending on the formulation, reactive polyurethane hot-melt adhesives cure to form elastomers with flexible to hard properties and tough adhesive layers. The prepolymers typically have a low free isocyanate content.
Non reactive solvent borne and water borne adhesives typically consist of a hydroxyl terminated polyurethane dissolved in a solvent. The polyurethanes are usually obtained by reacting a diol with a diisocyante. The polymer solutions are applied to both substrate surfaces to be bonded, some time is allowed for the solvents to evaporate and the surfaces are bonded together, at which point interdiffusions of the polymer chains will occur.
Non-reactive hot melt adhesives typically consist of linear chains that are solid at room temperature and are often used in the lamination of textiles although they have many other applications. They usually consist of hydroxyl-terminated polyurethanes that form the adhesive bond by cooling from the molten state. In some cases these are also known as thermoplastic polyurethane adhesives.
Polycarbonate polyols are available commercially in the polyurethane field. However, the commercial materials differ in structure from those used in the invention described below. Commercial polycarbonate polyols are all derived from diols (such as 1,4 butane diol, 1,6-hexane diol and the like) reacted with phosgene (or a reactive equivalent) to produce carbonate linkages between the diol units. No commercial polycarbonate polyols have only 2 carbon atoms between the carbonate linkages since synthesis of such materials is not possible with phosgene chemistry since the reaction results in formation of cyclic carbonate rather than polymer. The processes to make these polyols are also not particularly green. Phosgene is toxic, the diols are generally expensive and energy intensive to make and even non-phosgene-based process are energy intensive and expensive to operate.
Existing polycarbonate polyols, though expensive, are recognized to have excellent strength and resistance to hydrolysis and UV radiation and are therefore used where high performance is required. Nonetheless, there remains a need for inexpensive, green alternatives to these materials.